The invention relates to a gas generator, in particular a gas generator for air bags.
Gas generators for air bags have an ignitable ignition charge in a combustion chamber and a pressurized gas in a storage chamber. A gas or a solid fuel can be provided as ignition charge. Upon the initiation of the gas generator, an ignition element ignites the ignition charge. To open the storage chamber an impact element in the form of a piston is accelerated by the pressure produced during the combustion of the ignition charge, with the piston destroying a closure element of the storage chamber. In this way, the pressurized gas in the storage chamber can flow out through the opened outlet into the air bag. Moreover, the combustible gases produced as a result of the burning-away of the ignition charge flow into the storage chamber, where they mix with the pressurised gas. Therefore, after the destruction of the closure element, first of all cold pressurised gas and subsequently a mixture of pressurised gas and combustible gas flow into the air bag. In this way, hot combustible gases are prevented from the outset from arriving in the air bag.
The pressure characteristic of known gas generators is predetermined by the overall construction, with the air bag which is connected to the gas generator in each case being inflated quite suddenly.
The object of the invention is to create a gas generator which can be easily varied in order to achieve different pressure characteristics.
The gas generator in accordance with the invention has, for opening a closure element of the storage chamber, an impact element constructed as a tube and having several openings. The openings are arranged in such a way that, after the destruction of the closure element, at least one admission opening is arranged inside the storage chamber and at least one exit opening is arranged outside the storage chamber. In this way, after the destruction of the closure element, gas flows out of the storage chamber, through the admission opening into the tube and through the exit opening out of the tube into the air bag. The mass flow of the gas flowing through the openings can be influenced by the formation of one or more openings. Therefore, the mass flow can be influenced by the shape, the number and the arrangement of the inlet and/or exit openings. During the displacement of the tube upon the destruction of the closure element, a part of an entry opening or an exit opening can be closed and in the end position of the tube it can be completely open. In this way, the emergence of gas at the beginning of the inflation of the air bag is less. This avoids injuries to a driver or passenger by sudden inflation of the air bag.
By using differently shaped tubes as impact element, by varying a single component, gas generators can be manufactured with different pressure characteristics. Because the tube is a simple component, different formations of the tube can be produced easily and economically. Tubes with different openings which generate different pressure characteristics can be used in an otherwise unchanged gas generator. A change of the openings of the tube does not make necessary any changes on the rest of the assembly of the gas generator.
To change the temporal sequence of the pressure build-up, in a preferred embodiment of the invention the front of the tube which faces the closure element is closed. The exit openings are arranged on the circumference of the tube. Before the destruction of the closure element by the tube, the exit openings are arranged within the storage chamber. Upon the displacement of the tube for the destruction of the closure element, the exit openings are closed at least partially at the beginning of the displacement by a wall of the storage chamber. The further the tube exits from the storage chamber, the more the exit openings of the tube are opened. In this way, the pressure increase in the air bag at the beginning of the inflation is relatively small and increases continuously as the exit openings become larger.
In order to vary the temporal pressure characteristic, several exit openings can be staggered with respect to each other in the longitudinal direction of the tube. In particular, the exit opening can be constructed as a slot.
Several admission openings can be provided in the tube, so as to correspond with the exit openings, distributed on the circumference. The admission openings are preferably offset with respect to each other in the longitudinal direction in order to improve the flow performance upon the entry of the gas into the tube.
In another independent embodiment, a piston is provided in place of the tube, to control the pressure characteristic. The piston is formed in such a way that an exit gap is formed between the piston and the opening, closed by the closure element, after the destruction of the closure element, the cross-sectional area of which exit gap changes in dependence upon the position of the piston. If, for example, a tapering piston is used, the mass flow issuing from the gas generator increases with the displacement of the piston. The shape of the piston can be arbitrarily selected according to the desired pressure characteristic.
Both with the impact element constructed as a tube and with the impact element constructed as a piston, the pressure characteristic can be further optimized by the provision of a damping element. In this respect, the damping element can be deformable in an elastic or plastic manner. Thus, by means of an elastic damping element, for example a spring, the exit opening in the end phase of the inflation of the air bag may again be partially closed in order to reduce the strain on the air bag.
To dampen the tube or the piston, it can, moreover, plunge into a recess. In this respect, the intensity of the damping depends on the gap width between the recess and the tube or piston.
A further possibility of changing the pressure characteristic is the arrangement of at least one additional ignition composition in the ignition charge. The additional ignition composition or compositions can be ignited in a time-staggered manner with respect to the first ignition composition which activates the gas generator. The ignition of another ignition composition effects an increase of the pressure in the storage chamber and therefore a change of the pressure characteristic in the air bag. The ignition of further ignition compositions can also be made dependent on external parameters, for example the vehicle speed or similar.